Optical networks increasingly use wavelength division multiplexing (WDM) as a method to increase bandwidth. Multiple optical channels are combined and transmitted simultaneously as a single multiplexed signal. At the receiving end a demultiplexer separates the channels by wavelength and routes individual channels.
Optical amplifiers are commonly used in optical communication systems as in-line amplifiers for boosting signal levels to compensate for losses in a transmission link. In WDM systems, optical amplifiers are particularly useful because of their ability to amplify many optical channels simultaneously. For example, rare earth doped fiber optical amplifiers, such as erbium doped fiber amplifiers (EDFAs), semiconductor optical amplifiers (SOAs) and/or Raman optical amplifiers (ROAs) are used extensively for these purposes.
ROAs show potential over some of the other optical amplifiers in that they exhibit a high transparency when not pumped, because they can utilize silica-based fiber as a gain medium, and because they are suitable across the entire optical communication transmission window of silica optical fibers.
ROAs are typically referred to as either distributed Raman amplifiers, wherein the gain medium is part of the transmission fiber, or discrete Raman amplifiers, wherein the gain medium is a coil of optical fiber. In both amplification systems, the ROA includes a pump source for launching a pump beam into the gain medium. Raman gain is achieved through stimulated Raman scattering (SRS), which is an inelastic scattering process in which an incident pump photon loses its energy to create another photon of reduced energy at a lower frequency. The remaining energy is absorbed by the fiber medium in the form of molecular vibrations.
ROAs are typically designed to have a dynamically adjustable output power level. In particular, using a pump with a dynamically adjustable output power level allows a ROA to maintain constant gain for optical signals whose powers and numbers vary due to switching and routing, network reconfigurations, failures, and/or recovery from failures.
The dynamic range of a ROA pump is of particular importance in dynamically reconfigurable networks, where signal channels are re-routed in real time between any two nodes within the network. This requires an ROA at each node to be able to provide a large dynamic gain range. For example, consider a fiber span between node A and node B with 20 dB loss over about 100 km. During normal operation, signal channels are transmitted from node A to node B and an ROA at node B provides 17 dB of on-off gain. When a demand occurs, the network is reconfigured to handle signal transmission from node C to node B where the fiber loss is only 3.5 dB over about 17 km. In this case, in order to keep the signal level at node B constant, the Raman amplifier should only provide 0.5 dB of on-off gain. This corresponds to a required pump power dynamic range of more than 15 dB.
In addition, the dynamic range of a ROA pump is important when used in ROAs having multiple pump sources. Multiple pump sources are frequently used in ROAs to increase the bandwidth and/or generate a relatively flat gain curve. In particular, two or more pump sources are used frequently to provide pump energy at two or more pump wavelengths such that a relatively flat composite gain spectrum is obtained. In some instances, it is necessary for one or more of these pump sources to be operated at low, or very low, pump power levels. For example, if a large number of pump sources is used, the pump power of each pump source will need to be reduced accordingly. Alternatively, low power levels are used to provide slight shifts in the gain curve from at least one of the pump sources.
Unfortunately, most ROA pump sources are fiber Bragg grating (FBG) stabilized semiconductor diode lasers and are not typically designed to provide a large dynamic range and/or to operate at low powers. In particular, the ROA pump sources typically have associated therewith a minimum operating power level, below which, grating stabilization of the lapsing wavelength may not occur, or may become unstable, causing wavelength and power fluctuation.
Grating stabilization occurs when light is fed back into the laser, for example from a fiber Bragg grating (FBG), so as to perturb the coherent optical field formed in the diode laser cavity. This perturbation interrupts the coherence of the laser emission, which is referred to as coherence collapse, broadening the bandwidth of the laser emission by several orders of magnitude and resulting in multiple longitudinal mode operation of the laser source.
Advantageously, grating stabilization effectively locks the laser cavity output to the fixed wavelength of the grating and centers the external cavity multi-longitudinal modes around that wavelength, and thus fluctuations in wavelength of the laser source, such as those caused by changes in temperature or current, are eliminated. In addition, grating stabilization reduces the magnitude of mode-hopping noise in the laser (due to the presence of the multi-longitudinal modes), reduces perturbations from extraneous optical feedback from reflective components located beyond the grating, and permits the laser to deliver high optical powers into the fiber without the onset of stimulated Brilloum scattering (SBS).
Unfortunately, when grating stabilization fails (such as below the minimum power level discussed above) the laser will fall back into single longitudinal mode, and the majority of the light provided by the laser will be reflected back from the transmission fiber through SBS. In other words, the dynamic range of a FBG stabilized ROA pump is limited, at least at the lower end, by the minimum power level needed to maintain coherence collapse of the laser.
In U.S. patent application Ser. No. 10/269,925 filed Oct. 11, 2002, hereby incorporated by reference, Ratoff et al. disclose a method of increasing the dynamic range of pump sources used in rare-earth doped amplifiers, such as erbium doped fiber amplifiers (EDFAs). In particular, Ratoff et al. teach a method of modulating an EDFA pump source via pulse width modulation. The purpose of this modulation is to extend the dynamic range of the EDFA pump source such that channel dropping/adding does not significantly affect the EDFA's power output for each channel. Notably, this problem is more significant for EDFAs than for ROAs because EDFAs are more easily saturated.
In the prior art, modulated (or dithered) pump sources have been also proposed for use in rare-earth doped amplifiers to remove kinks from power versus current curves (see U.S. Pat. No. 6,240,119 hereby incorporated by reference), decrease susceptibility to feedback (see U.S. Pat. No. 5,499,135 hereby incorporated by reference), remove undesirable perturbations in the laser source output (see U.S. Pat. No. 6,215,809, hereby incorporated by reference), and allow the generation and launching of higher than expected power optical signals without experiencing the effects of SBS (see U.S. Pat. No. 5,477,368 and, for comparison, U.S. Pat. Nos. 6,331,908 and 6,516,113, hereby all incorporated by reference).
Notably, none of these references propose a system or method for improving the dynamic range of Raman pumps, which for example, needs to be increased due to the dynamic routing of signal wavelengths, and/or using low pump powers, which for example, is useful when using multiple pumps.
Modulated pumps, however, have been used in Raman amplifiers to reduce pump interactions in the fiber arising from the use of two different pumps (see U.S. Pat. No. 6,456,426 hereby incorporated by reference), to provide a closed loop control by providing a small amount of low frequency modulation to the pump (see U.S. Pat. Nos. 6,373,621 and 6,452,716 hereby both incorporated by reference), and to provide a degree of randomness to the pump signal to minimize cross talk between signals and improve the transfer of power from the pump to the signal (see U.S. patent application Ser. No. 09/990,206 filed Nov. 21, 2001).
It is an object of the instant invention to provide a Raman amplifier having an improved dynamic range.
It is a further object of the instant invention to provide a FBG stabilized pump laser source for a Raman amplifier that has a broad range of output power.
It is yet another object of the instant invention to provide a FBG stabilized pump laser source for a Raman amplifier that can provide stable output at very low power levels.